Cells of micro-algae (in short “algae”) are rich in various bioactivity substances such as proteins, amino acids, carbohydrates, vitamins, antibiotics, highly unsaturated fatty acids, polysaccharides, and colorants. This makes algae great resources with high economic value. Some algae possess abilities to produce hydrocarbons and oil lipids, and thus have promising application in field of renewable energy production. For example, algae lipids can be processed into biodiesel and/or jet-fuel (third generation feedstock for Biofuel), sugar and hydrocarbons into ethanol, and potentially into hydrogen, methanol and bio-power, while the residual biomass (such as proteins, pigments, etc.) can be used for pharmaceuticals, neutraceuticals, cosmetics, fishmeal, biochar, or other applications.
Algae can double in volume overnight and can be continuously harvested on a daily basis. Algae need sunlight, carbon-dioxide, water, nutrients and temperature for their growth. The algal cells fix carbon-dioxide through photosynthesis and carbon usually comprises more than half of its dry weight. Therefore, sufficient carbon source and sunlight are needed during algae cultivation.
There are two main types of large scale algae biomass production systems: open and closed (photobioreactors).
Open systems can be categorized into natural waters (lakes, lagoons, ponds) and artificial ponds or containers. The most commonly used systems include shallow big ponds, tanks, circular ponds, raceway ponds and high-rate ponds (HRP). One advantage of open ponds is that they are easier and cheaper to construct and operate than most closed systems. However, a limitation in open ponds includes poor light utilization by the cells, evaporative losses, diffusion of CO2 to the atmosphere, and requirement of large areas of land. Furthermore, contamination by viruses, fungi, predators and other fast growing heterotrophs have applied some restriction on the commercial production of algae in open culture systems to essentially only those organisms that can grow under extreme conditions. Also, due to low-efficient stirring mechanisms in open cultivation systems, their mass transfer rates may be very poor resulting in low biomass productivity.
The closed photobioreactors have different structures, such as airlift reactor, stirred reactor, flat panel or tubular reactor, which can be used for producing high value added products (such as medicinal or health products) or used as seed tank for open-pond cultivation, mainly due to very high costs (capital and operational).
Hybrid systems attempt to mix the best qualities of open and closed systems in order to achieve economic competitiveness. Usually, small closed systems grow a preferred algae species, which then seed a large open system. The higher fixed and operational costs of the closed system are kept to a minimum by keeping its relative size small, while the risk of environmental exposure of the open system is minimized by seeding it with sufficient amounts of algae from the closed system so that the preferred species dominates the pond.
Belgian Patent No. BE 1011897 describes algae cultivating devices comprising, inter alia, a support for holding angled transparent walls, gas and liquids inlets and reflectors. The devices can be connected to one another in series or parallel to form assemblies.
U.S. patent application Nos. 2008/0311649 and 2009/0215155, in the name of XL Renewables Inc. describe an apparatus for producing algae, the apparatus comprising flexible reactor tubing that has a wall that is at least partially translucent to sunlight; and a stationary circulation pump for moving algae fluid through the reactor tubing. The reactor tubing is made of clear polyethylene with UV inhibitors and preferably has a substantially circular cross-section that lies flat when not pressurized. Gas relief valves allow gases generated during algae production to escape from the reactor tubing. CO2 may be injected into the algae fluid to stimulate photosynthesis.
International Patent Application Publication No. WO2004/008845 describes a culture tank comprising a receptacle for containing a culture medium and organisms to be cultured and substantially flat filter means arranged to filter particulate material from the culture medium; fluid motion imparting means arranged in use to introduce fluid into the receptacle such that the culture medium in the receptacle is urged to flow in a direction substantially parallel to the filter means so as to minimize collisions between organisms and the filter means; and additional means for urging the culture medium near the filter means to flow through the filter means.
Finally, U.S. Pat. No. 5,158,037 describes a device for aquaculture especially for raising fish, plankton and algae, with addition of oxygen-containing gas and nutrients, with aerating devices arranged in pools or ponds of a suitable shape and a gas through-put adjusted so that a bubble curtain is produced without major turbulence in the water of the pond. The bottom portion of the pools or ponds may be tilted towards the aerating device and the aerating device located at the deepest point of the pool/pond. As a result, the bottom is either flat or V- or multiple V-shaped, pyramidal or conical, with aerating devices located at the deepest point.